Motor control devices having controlled "soft start" and "soft stop" function wherein the motor is protected from overcurrent surges during both starting and stopping operations is described within U.S. Pat. No. 5,341,080 entitled "Apparatus and Three Phase Induction Motor Starting and Stopping Control Method".
Motor control devices having improved thermal protection for the semiconductor switches used to control the motor start and stop functions are described in U.S. patent application Ser. No. 08/904,293 filed on Jul. 31, 1997 entitled "Compact Electrical Equipment Enclosure".
Means are employed to prevent an electric motor from being turned on for a fixed period of time after turn-off in order to protect the motor and associated electrical equipment from thermal damage. One such arrangement is described within U.S. Pat. No. 3,815,668 entitled "Motor Controller Unit" to protect a cooling system compressor from being turned on for a two minute period after turn-off to protect the motor.
U.S. Pat. No. 3,854,631 entitled "Motor Starter Unit" describes a lockout counter to prevent motor restart after a predetermined number of motor starts to protect the electric motor and associated electric equipment from thermal damage.
An early attempt to protect a motor from overheat damage during start-up is described in U.S. Pat. No. 4,476,423 entitled "Motor Control Apparatus with Motor Starts Per Time Period Limiter" wherein the number of starts over a given time period is stored in memory. A minimum time period is calculated and the motor is prevented from start-up until the time period is exceeded.
It has since been determined that such semiconductor switches such as SCR's, when used within each pole of multi-pole electric motor to control the starting and stopping function can become subjected to thermal damage when an attempt is made to re-start a motor while the SCR's are warm from previous operation.
Since the cool down rates for electric motor windings and SCR junctions are not the same, a predetermined start inhibit signal to protect motor windings within electric motors exterior to a motor controller cabinet may not be sufficient to prevent thermal damage to the SCR's used within the motor controller unit contained within the cabinet.
In the absence of heat sinks and cooling vents to control the temperature of the SCR's as described in the aforementioned U.S. patent application Ser. No. 08/904,293 such as used within compact motor controller units, it would be beneficial to utilize the SCR's during motor start-up and by-pass the SCR's during motor operation to keep the heat generated within the SCR's to a minimum.
Accordingly, one purpose of the invention is to describe a simple and inexpensive approach to ensuring protection to the motor controller unit SCR's independent from the means employed to prevent thermal damage to the motor windings, per se and in the absence of heat sinks, cooling vents and the like.